This invention relates to a disposable hypodermic syringe, which can be filled and used once, and which is designed in such a manner that re-use of the syringe is rendered impossible. The design of the present invention provides a reliable and effective way of reducing the risk of contamination when hypodermic syringes are re-used without proper sterilization. In addition, use of this hypodermic syringe would serve to curtail the spread of infectious diseases by intravenous drug users, who are known to share and re-use syringes amongst themselves.
A number of designs for non-refillable hypodermic syringes are available in the prior art. In one such design, disclosed in U.S. Pat. No. 4,650,468, the user of the syringe must twist the plunger assembly after injection is completed in order to retract the plunger and to appropriately position a piston housing assembly to engage a locking mechanism which ensures that the piston may not be moved from its retracted position. However, this type of design may not render the syringe useless after the first injection if the plunger assembly is not intentionally twisted and retracted to cause the requisite locking action. Additionally, the overall design and the number of components necessary to assemble this prior art syringe makes it overly complex and expensive to manufacture.
In another type of prior art design disclosed in U.S. Pat. No. 4,687,467, a needle extends rearwardly into the syringe and provides a sharp cutting edge which punctures the end of the piston during the final forward movement of the piston during injection. Here again, the syringe may be re-used if the piston is not pushed all the way forward to effect the puncturing. Other prior art designs also suffer from the same problem wherein someone determined to re-use the syringe may do so by circumventing the built-in safety mechanisms.